Can you get thumbnails in SKYBOX PC Client? If you can't see the thumbnails in SKYBOX PC Client, please delete and re-add the file to see if it works. Btw, please click SETTINGS to check the version number.
Besides, we hope you can open SKYBOX on your Quest, click Global Settings(the gear icon), click the BUG icon in the upper-right corner, and send us a bug report attached with logs. You just need to tick the "upload logs" box then send feedback.
It would be better if you can share with us a video that doesn't have thumbnail. We'd like to test the video to further investigate the issue.